Add pkg-config support

[yegorich]

Adding pybind11.pc (https://www.freedesktop.org/wiki/Software/pkg-config/) to setup.py would greatly simplify the task of finding pybind11 via CMake.

[wjakob]

Does setup.py even have a mechanism to install .pc files into the right places? That said, this is not really a priority for me, and I don't consider it an issue (so I am closing this ticket).